RF power delivery systems provide power to dynamic loads typically at frequencies between about 400 kHz and about 200 MHz. Frequencies used in scientific, industrial and medical applications are approximately 2 MHz, 13.56 MHz, and 27 MHz. Depending on the application, RF power is delivered in a pulse and/or a continuous-wave mode to a load. Controlling delivered RF power has become increasingly important in semiconductor manufacturing as the dimensions of semiconductor features have continued to decrease. The ability to more precisely control RF power parameters enables a semiconductor manufacturer to achieve smaller semiconductor features. This is particularly difficult, however, when the RF power is delivered to dynamic loads.
Various approaches exist for controlling pulsed RF power that is delivered to dynamic loads. One approach is to use a look-up table of known operating parameters to control the amplitude and shape of delivered RF power on a pulse-by-pulse basis. Another approach is to use optimal, constant parameter estimates around a nominal operating point. A third approach is to use high-bandwidth and/or high-speed components (e.g., a power-sensing circuit, a digital signal processor, and/or a pre-regulator) to regulate the amplitude and shape of delivered RF power on a pulse-by-pulse basis.
Problems exist, however, with each of these known approaches. In the first and second approaches, performance can degrade when processing conditions change and/or drift from the values in the look-up table or the nominal operating point. In the third approach, high-speed components add significant cost to control systems. Moreover, the control system is susceptible to performance degradation due to the electrical noise associated with high gain and high bandwidth systems.
Various approaches exist for switching an RF power delivery system from a pulsed mode to a continuous-wave mode. One known approach is to use an open-loop system, where the input voltage to an RF power amplifier is fixed and pulses are generated by switching the RF power amplifier on and off. However, open-loop systems lack the ability to modify the delivered power based on changes in operating conditions at the load. Further, open-loop systems are unable to compensate for the high number of plasma oscillations that occur when using low-frequency pulses for plasma processing applications. Another known approach to switching between pulsed and continuous-wave power is to temporarily stop processing between power-delivery modes. However, temporarily stopping processing results in irregular processing after system start-up. Moreover, temporarily stopping processing results in an unstable plasma because the power is not constant. Finally, temporarily stopping processing increases processing and cycle time.